High voltage electrolytes for lithium-ion batteries with micro-sized silicon anodes

Micro-sized silicon anodes can significantly increase the energy density of lithium-ion batteries with low cost. However, the large silicon volume changes during cycling cause cracks for both organic-inorganic interphases and silicon particles. The liquid electrolytes further penetrate the cracked silicon particles and reform the interphases, resulting in huge electrode swelling and quick capacity decay. Here we resolve these challenges by designing a high-voltage electrolyte that forms silicon-phobic interphases with weak bonding to lithium-silicon alloys. The designed electrolyte enables micro-sized silicon anodes (5 µm, 4.1 mAh cm−2) to achieve a Coulombic efficiency of 99.8% and capacity of 2175 mAh g−1 for >250 cycles and enable 100 mAh LiNi0.8Co0.15Al0.05O2 pouch full cells to deliver a high capacity of 172 mAh g−1 for 120 cycles with Coulombic efficiency of >99.9%. The high-voltage electrolytes that are capable of forming silicon-phobic interphases pave new ways for the commercialization of lithium-ion batteries using micro-sized silicon anodes.

spectra were collected for all three samples for a detailed comparison.A co-axis NMR technique is applied with external C6D6 as a reference for the in-situ 1 H-NMR measurement.Supplementary Fig. 9a shows the full 1 H-NMR spectra of the three electrolytes together with the LiEMC standard dissolved in d 6 -DMSO.Note that the LiEMC itself undergoes an equilibrium reaction that happens in the d 6 -DMSO solvents, 11 such that only the signature LiEMC peaks at 3.67 ppm and 3.42 ppm were chosen for a clear comparison.Supplementary Fig. 9b shows an enlarged view between 3.8 ppm to 3.4 ppm for all samples, with the electrolytes signal intensity magnified by 100 times.The EE electrolyte shows ambiguous signals for the LiEMC, while no obvious signal was found for the FFT electrolyte, even after the 100 times magnification, which makes sense as the fluorinated of the carbonate electrolytes will reduce its solvation ability, such limits the solubility of organic species.On the other hand, for the FST electrolytes, an apparent signal could be identified as the LiEMC, though the signal somehow low-field shifted in the FST electrolytes compared to the d 6 -DMSO standard.Nevertheless, the result here clearly shows that the strong solvation ability of sulfolane molecules could indeed dissolve some of the organic components in the SEI.As such, only the insoluble inorganic species like LiF and Li2O will be accumulated in the SEI, which will guarantee a ceramic LiF-Li2O SEI that is designed for the success cycle of SiMPs.

Discussion of the SEI structure from different electrolytes
There are contrasting composition differences among the SEI formed in the three electrolytes, EE, FFT, and FST, with the outstanding ones being the C-and O-content as well as their trends during the Ar + sputtering, as illustrated in the main text.Supplementary Fig. 13 shows the atomic ratio of all elements found in the SEI layer cycled in different electrolytes.Obvious differences were observed for the C, O, and F signals: for μSi electrode cycled in EE and FFT electrolytes, the atomic composition remained the same from the surface to the inner part till 600 S sputtering, indicating an evenly distributed SEI with all species mixed, that is, the formation of organic and inorganic species happened at the same time with no preference in these two reference electrolytes.
The high content of C-and O-signals showed up at the same time in EE electrolytes implies organic fragment formation such as ROLi, LiOCOR, or Li2CO3, all of which show poor resistance of the large volume expansion during the cycling of the μSi anode.Though the F-content has been largely improved in the FFT electrolytes, the high ratio and evenly accompanying C-content signifies the organic-dominated SEI, with less amount of LiF embedded inside the organic matrix, resulting in a less uniform and less compact SEI.This causes more electrolyte penetration through the SEI and further electrolyte decomposition on the SiMPs surface, forming a thicker SEI, as proved by the thickness evolution test during cycling (Supplementary Fig. 24), all of which leads to increasing impedance in the EIS spectra (Supplementary Fig. 17b), low cycling CE (<99%) and quick capacity fading.For the designed FST electrolytes, the O-ratio showed a clear increase from the surface to the inner part of the SEI during the sputtering, along with a C-ratio decrease, indicating enrichment of inorganic Li2O, and a large decrease of organic species.The more Li2O formation along with LiF in the SEI can be attributed to two factors: 1) more SL solvent decomposition due to the much stronger solvation ability of SL compared to FEC or TTE, as confirmed by Raman spectra (Supplementary Fig. 2a) and MD simulations (Supplementary Figs. 2, 4).2) the initial reduction of FEC helps to form the LiF-rich inner core, which favors the following reduction of SL to form Li2O. Two weak S2p signals at 170 eV and 163 eV were also observed in the FST electrolytes (Supplementary Fig. 12), which can be ascribed to Li2SO3/4 and Li2S, indicating the complete reduction of the SL molecule, leaving limited organic species in the SEI.
LiF (Supplementary Fig. 15).More specifically, the dominant diffusion carrier in Li2O is Li-ion interstitial, while in LiF is Li-ion vacancy from Schottky defects owing to lower formation energy.Therefore, lattice Li-ion in LiF will spontaneously migrate towards Li2O lattice to form interstitial defect as evidenced by the negative Gibbs free energy of the defect reaction in the Li2O-LiF interface (see Method part below for more details).In this model, the topological distribution of the LiF and Li2O phases was simplified as alternatively parallel so that the Li + conduction path could penetrate along the SEI (Supplementary Fig. 16a).The simplified model provides an upper limit estimation of the ionic conductivity in actual SEI, where the tortuosity factor also affects Li + conduction significantly.The defect reaction was found to boost the interstitial Li + defect concentration in the Li2O lattice near the LiF-Li2O interface up to 10 4 times and reduce the electron concentration to 10 -4 compared with that of bulk Li2O.(Fig. 6e, main text) According to the space charge model, when only a 5% volumetric percentage of LiF was added to Li2O with a grain size of 15 nm, the ionic conductivity of SEI increased from 3.0*10 -5 mS cm -1 in pure Li2O to 2.0*10 -3 mS cm-1 in Li2O-LiF composite.(Fig. 6f, main text) Further reducing the grain size of Li2O and increasing the amount of LiF can generate more Li2O-LiF interface and improve the contribution of space charge effect to total conductivity.In this work, the grain size of Li2O was estimated to be < 3nm based on TEM observation (Supplementary Fig. 14) and the volumetric percentage of LiF is about 18% from XPS spectra (Supplementary Fig. 13, more details in Method part 3 below).

Identifying the dominant point defect of LiF and Li2O
Dominant Li point defect of LiF and Li2O at the Li anode was identified based on defect formation energy calculation.The formation energy of point defect X with charge q is (1): Where   [  ] and   [] are the DFT total energy of LiF or Li2O supercell with and without defect.  is the number of Li atoms (ions) added to or removed from the supercell.µ  is the chemical potential of Li.   is Fermi level referenced to the valence band minimum (VBM).
By requiring overall charge neutrality, the Fermi level can be determined from equation (2)      Where   (  ) is the number of sites and defects  can be generated per unit volume.  and   are the integrated density of states (DOS) of the conduction and valence band.  is the bandgap of LiF or Li2O bulk supercell.Temperature  is 300 K.   is Boltzmann's constant.The calculations were performed with the help of Pymatgen 13 and Pycdt 14 software.

Defect distribution and ionic conductivity in space charge region
The space charge model was set up as previously reported. 13Possible Li point defects reactions in the interface of LiF and Li2O were listed as follows: Here, µ is the reaction free-energy of point defects and was calculated based on defect formation energy from DFT calculations and the reported data of published papers by the same author for consistency. 15We can see, only reaction (1) has a negative chemical potential difference, which can occur spontaneously.That is, when Li2O is in contact with LiF, Li-ion will leave the LiF lattice and accumulate in the Li2O lattice as interstitial defects.The concentration of interstitial Li + defect and electron in Li2O lattice near the Li2O-LiF interface can be described by equation (3): Where   is the concentration of Li + interstitial defect in bulk Li2O material.Parameter k is a constant and is equal to Where   is the size of the Li2O grain.  * is the Li + interstitial defect concentration in the center of the Li2O grain, which can be obtained from Equation 3.   is calculated from Equation (5): 17 Here,  is the lattice vibration frequency (=10 13 Hz). is the hopping distance of interstitial Li (= 4.65 Å).   is the migration barrier of Li + in Li2O. 17

Volumetric ratio of LiF and Li2O in composite SEI
In LiF-Li2O composite SEI, the volumetric ratio of X (X = LiF or Li2O) in SEI can be calculated by equation (6): The molar volume    can be calculated by    =     , where   is the molar mass (LiF is 25.93 g mol -1 and Li2O is 29.88 g mol -1 ). is the volumetric density (LiF is 2.635 g cm -3 and Li2O is 2.013 g cm -3 ).  is the molar ratio of LiF and Li2O.Based on XPS, they are   :  2 ~ 1 : 3. Plug in all numbers we get   = 18%.
] 0.5 .Here,   =0 was determined by   =0 =   (  ⦁ ){−(  [  ⦁ ] + µ)/  }.  is Debye length which defined as   = (    2 *  2   ) 0.5 . and   are vacuum and relative permittivity, respectively. is the ideal gas constant. is the Faraday constant.Plug in all parameters and we get the Debye length of Li2O is 2 nm.The normalized concentration profile of interstitial Li + and mobile electron Li2O within the Li2O-LiF space charge region is shown in Fig. 6e (main text).The total ionic conductivity of the LiF-Li2O composite SEI as a function of the volume fraction of LiF was calculated based on Equation (4) 16 :